1. Field of the Invention
The present invention relates to a sheet folding device for folding a sheet ejected from an image forming apparatus such as a copier and a printing machine and an image forming apparatus provided with the sheet folding device, and an image forming apparatus provided with the sheet folding device.
2. Description of the Related Art
When a desired fold line is to be formed on a sheet with a fold applied by a folding roller so as not to lose productivity, it is general to cause the sheet with a folded portion to pass through a nip portion between fold-enhancing rollers to enhance the fold. However, if the nip portion between the fold-enhancing rollers is to be pressurized in order to form the desired fold line on the sheet, because of a mechanism in which the pressure is applied to both sides of a shaft of the roller, there arises a problem that the mechanism causes a deviation in a pressure distribution. Moreover, due to unevenness in a shape of a leading edge of the sheet including the folded portion, there occurs a phase difference in the way to cause the sheet to enter the nip portion of the fold-enhancing roller. Therefore, there occurs a problem that undesired deflection occurs in the sheet and the fold is enhanced in that state, to cause wrinkles in the sheet. Thus, a fold-enhancing mechanism is needed to form a desired fold line on the sheet applied with the fold, not to cause undesired deflection as a factor of wrinkles, or to correct the deflection.
To solve the problems, there is disclosed a technology for a pair of folding rollers that is provided with a nip portion formed over a whole width along an outer periphery of at least one of the rollers and also provided with a cutout-formed portion in which only a central portion of the other roller is formed as a nip portion and the other portions thereof are formed as cutout portions. In the pair of folding rollers, a fold is applied to a sheet in the nip portion as the former portion and a deflection is released while the sheet is conveyed by the cutout-formed portion as the latter portion (e.g., Japanese Patent Application Laid-open No. 2001-019270).
However, the technology described in Japanese Patent Application Laid-open No. 2001-019270 has a problem as follows. Because the nip portion to apply the fold thereto is formed over the whole width, a pressurizing force applied to the sheet is dispersed, so that a satisfactory fold line cannot be formed on the sheet. Moreover, there is also a problem that because it is necessary to match a timing of the nip portion in the roller and a timing at which the sheet enters the nip portion, control is complicated, and this leads to difficulty in achievement of high productivity.
On the other hand, to solve the problem on dispersion of pressure of the fold-enhancing roller, there is disclosed a technology for providing a plurality of fold-enhancing roller pairs whose roller width is smaller than a sheet width, arranging the roller pairs in such a manner that the roller pair is disposed at the center of a fold line of the sheet in an upstream side and the roller pairs are disposed gradually outward of the fold line of the sheet along the downstream side to enhance the fold over the whole sheet, and forming the fold line on the sheet (e.g., Japanese Patent Application Laid-open No. 2007-045531).
However, the technology described in Japanese Patent Application Laid-open No. 2007-045531 has problems as follows. Because the fold at the central portion of the sheet is first enhanced by the fold-enhancing rollers with a high pressurizing force, the both sides of the sheet move slightly upward. Therefore, a phase difference occurs in the way of causing the leading edge of the sheet to enter a nip portion of the fold-enhancing rollers that enhance the fold in the outside of a subsequent sheet to enter, and this causes a deflection in the sheet, so that wrinkles easily occur therein.
As explained above, the technology described in Japanese Patent Application Laid-open No. 2001-019270 has the problems that the pressurizing force applied to the sheet is dispersed and the high productivity is difficult to be obtained because of the complicated control. Furthermore, the technology described in Japanese Patent Application Laid-open No. 2007-045531 has the problems that although the dispersion of the pressurizing force applied to the sheet is prevented, the deflection occurs in the sheet and the wrinkles thereby easily occur.